Aftertreatment of diesel exhaust gases is used inter alia to reduce the nitrogen oxide content of exhaust gas. One known method for this purpose is “selective catalytic reduction” (SCR). In SCR, nitrogen oxides are reduced to water and nitrogen over a catalyst by the reducing agent ammonia. Conventionally, the ammonia is introduced in the form of a urea solution (e.g., AdBlue) into the exhaust tract ahead of the SCR catalyst. A catalyst is ideally provided for hydrolysis.
If urea solution is introduced into the exhaust tract, ammonia and isocyanuric acid are formed from urea by thermolysis, and then ammonia and carbon dioxide are formed from the isocyanuric acid by hydrolysis.
However, the inventors herein have recognized potential issues with such approaches to the aftertreatment of diesel gases. As an example, the above-discussed methods may require sufficiently high exhaust gas temperatures for the reaction to proceed in an optimum manner. For example, the thermolytic-hydrolytic conversion of the urea-containing reducing agent does not work at low temperatures. Further, the SCR reaction is also temperature-dependent. Below 150° C., there may be virtually no reaction, while an almost 100% reaction takes place above 220° C. Moreover, reaction efficiency depends on the ratio of the various nitrogen oxides. Under conditions with low exhaust gas temperatures, e.g., in urban traffic, conversion efficiency in the SCR catalyst is relatively low. Even when the temperature of exhaust gas released from the engine is high, due to the position of the SCR catalyst downstream of one or more other exhaust catalysts (e.g., as the second or third element in an exhaust tract), thermal energy of exhaust gas may be lost between the combustion engine and the SCR catalyst. As a result, the exhaust reaching the SCR catalyst may not be hot enough.
It is therefore the object of this disclosure to provide a sufficiently high temperature for the decomposition of the reducing agents and the progress of SCR.
A first aspect of the invention relates to a system for exhaust gas aftertreatment of a combustion engine in a motor vehicle comprising, an exhaust tract; a first catalyst device arranged in the exhaust tract, a second catalyst device arranged downstream of the first catalyst device in the exhaust tract; a feed device for injecting a reducing agent arranged in between the first and the second catalyst device; a first heat device arranged at an inlet of the first catalyst device and a second heat device arranged downstream of the first catalyst device; and a control device.
Advantages of this system may include the ability to increase the temperature of the exhaust gas in accordance with the operating situation. As a result, the operation of the catalysts can begin immediately after the starting of the combustion engine. In addition, rear-injection of a urea solution onto a warm surface, such as a rear face of the first catalyst, as opposed to injection further downstream at lower temperatures, increases decomposition of exhaust gas emissions. Further advantageous embodiments of the invention will become apparent from the additional independent claims and dependent claims, the description, the figures and the illustrative embodiments.
In the present application, the term “reducing agent” is also used for a precursor of the reducing agent, e.g., for urea or an aqueous urea solution, even if the actual reducing agent is ammonia formed from urea in a conversion reaction. In other words, the reducing agent is provided indirectly.
The term “catalyst devices” is used to denote technical units which comprise at least one catalyst. It is possible for a plurality of catalysts to be arranged in a catalyst device. For example, parts of the matrix of the catalyst device are coated with a catalytically active layer, for which reason the term “a coating with a catalyst” is also used in this application.
In one example, the first heat device is arranged at the inlet of the first catalyst device. The first heat device is advantageously used to heat the exhaust gas entering the first catalyst device in order to reach the operating temperature of an oxidation catalyst or is used indirectly to heat the SCR catalyst. In this way, exhaust gas at the SCR catalyst may be made sufficiently hot irrespective of a position of the SCR catalyst with reference to other exhaust catalysts in the exhaust tract.
In another example, the first heat device is coated with the oxidation catalyst. By virtue of the necessary temperatures being reached via the first heat device, hydrocarbons and carbon monoxide can be oxidized over the oxidation catalyst within a short time of starting the combustion engine. A lean NOx trap (LNT) may also be arranged in the first catalyst device. The LNT is used to store nitrogen oxides during the operation of the combustion engine with a lean fuel mixture. When the stored nitrogen oxides are released in rich-mixture operation, the nitrogen oxides are released and reduced over the SCR catalyst. An SCR catalyst may be arranged in the first catalyst device, advantageously at the downstream end, i.e. at the outlet.
The second heat device may be arranged at the outlet of the first catalyst device as well. The second heat device is advantageously used to heat the exhaust gas in order to reach the operating temperature of the SCR catalyst or is used directly to heat the SCR catalyst to the operating temperature thereof.
If the second heat device is arranged at the outlet of the first catalyst device, it may be coated with a catalyst for selective catalytic reduction and/or with a hydrolysis catalyst. This arrangement advantageously allows the reduction of nitrogen oxides released from the LNT since the necessary temperature for thermolysis of urea can therefore also be produced for the operation of the SCR catalyst by virtue of the second heat device. Apart from the function of reducing nitrogen oxides, the SCR catalyst also has a hydrolysis function, thus making it possible, for example, to convert urea into ammonia and use it to further reduce nitrogen oxides. A separate hydrolysis catalyst can also be arranged in addition to the SCR coating.
The second heat device can also be situated further downstream, between the first and the second catalyst device. The SCR coating may be applied to the arrangement of the second heat device both in the first catalyst device and in the exhaust tract between the first and the second catalyst device.
In a further example, in the system according to the invention, at least one mixer is arranged between the first and the second catalyst device. The mixer is advantageously used for uniform distribution of reducing agent introduced into the exhaust tract, thus avoiding deposits in the exhaust tract, especially in the case of urea. Furthermore, the mixer enables reducing agent to be fed in at the outlet of the first catalyst unit, thus ensuring that it is available there for reducing nitrogen oxides over the SCR catalyst. In this way, reducing agent may be injected into the first catalyst unit in a rear facing direction, opposite to the direction of exhaust flow through the first catalyst unit.
A second aspect of the invention relates to a motor vehicle having an exhaust system according to the invention described above. The motor vehicle according to the invention thus has a system having an exhaust tract, in which at least one first catalyst device and one second catalyst device, arranged downstream of the first catalyst device, and a feed device for a reducing agent, which is arranged between the two catalyst devices, are arranged, and a control device, in which system a first heating device is arranged at the inlet of the first catalyst device and a second heating device is arranged downstream thereof.
A third aspect of the invention relates to a method for exhaust gas aftertreatment for the reduction of nitrogen oxides by virtue of a system according to the invention, comprising: starting the combustion engine, then switching on the first heating device, then introducing the reducing agent when a first temperature threshold value is reached, and then switching on the second heating device.
The advantages of the method correspond to those of the system according to the invention.
In one example of the method according to the invention, the second heating device is switched off in a further step if a second temperature threshold value is reached. The second temperature threshold value is reached when sufficiently high exhaust gas temperatures for the thermolytic-hydrolytic conversion and SCR function are achieved. In this case, additional heating by virtue of the second heating device may not be necessary.
In yet another example, the first heating device may be switched off if the second heating device is switched on. However, it is also possible for the first heating device to remain switched on while the second heating device is switched on.